Energy harvesting is the process by which energy is derived from external sources, captured, and stored for small, wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. A direct current to direct current ‘DC-to-DC’ converter circuit is an electronic circuit which converts a source of direct current (DC) from one voltage level to another by first charging a energy storage element using an input voltage from the, energy harvester and then discharging the energy storage element in order to provide the energy at the output of the DC-to-DC converter. It is a class of power converter. DC-to-DC converters can be used to increase the amount of energy harvested from an energy source. The maximum power-point (MPP) is the input voltage of the DC-to-DC converter at which the maximum amount of power is harvested. Maximum Power-Point Tracking (MPPT) is an algorithm or procedure which makes sure that the harvester is operated at or close to the Maximum Power Point.
A known way of doing MPPT is called linear approximation, wherein the harvester output voltage is regulated at 50% (in case of Piezo or Thermal electrical Generator (TEG)) or at 70% . . . 80% (Photovoltaic) of the harvester output open-circuit voltage Voc. It is known to sample or measure the harvester output open-circuit voltage Voc and multiply that voltage by a constant factor (0.5 for TEG or Piezo and 0.7 . . . 0.8 for Photovoltaic). The DC-to-DC converter attached to the harvester can be controlled in a way that the input voltage of the DC-to-DC-converter is regulated to that value. A drawback of this MPPT implementation is that it is necessary to sample the harvester output open-circuit voltage Voc on a regular basis (during that time the output power of the DC-to-DC converter is zero). This will lower the overall efficiency. Also a voltage dependent efficiency of the DC-to-DC converter itself is not taken into account; a voltage of 0.5×Voc is optimal for a TEG, but might not be the optimum for the combination of a TEG and a boost-converter. This is especially true for very low boost-converter input voltages, since the efficiency of boost-converters usually drops significantly at very low input voltages. Therefore, it is necessary to know the characteristics of the harvester (the MPP of the harvester relative to the harvester open-circuit voltage Voc).
Another way of doing MPPT is through “Perturb and Observe”, i.e., searching the peak of the power-curve by, for instance, making a small change (increase or decrease) to the input voltage Vin of the DC-to-DC converter and measure the power at the output of the DC-to-DC converter. If the power increases compared to the power at the previous input voltage, then continue changing Vin in the same direction (i.e., if Vin has been previously increased, then continue increasing Vin and vice versa). If the power has decreased, change Vin in the opposite direction (i.e., if Vin has been previously increased, then decrease Vin and vice versa). A difficulty of “Perturb and Observe” is an accurate measurement of the input voltage and current. For large solar panels this overhead in terms of cost and energy may be relatively small, but for small wearables and wireless sensors, the additional cost may be significant.
Bandyopadhyay, S.; Chandrakasan, A., “Platform architecture for solar, thermal, and vibration energy combining with MPPT and single inductor,” IEEE Journal of solid-state circuits, vol. 47, no. 9, September 2012, discloses a multi-input energy harvesting system using a time-based power monitor for achieving maximum power point tracking for the harvester wherein a boost-converter in fixed-frequency Discontinuous Conduction Mode (DCM) is used. The input impedance of this boost converter can be controlled by changing the amount of time T1 that the energy storage element is storing energy. The time T2 while the boost converter is discharging is a function of the power delivered by the harvester and the output voltage. The algorithm changes T1 and optimizes for the maximum value of T2. This approach has a drawback in that a DCM boost converter has its optimal efficiency for a limited range of peak-inductor-currents (for higher currents, resistive losses may become disproportionally large, whereas for lower currents the losses for closing/opening the switches may become disproportionally large). In the fixed-frequency approach, the peak-inductor current can vary over a very wide range. Therefore the boost-converter may often not have the optimal efficiency. Another limitation is that T2 has to be measured with a fine resolution.